In general, a dynamic random access memory (DRAM) is a memory device including a plurality of memory cells, each of which is composed of one transistor and one capacitor. In this DRAM, data can be written into or read from each memory cell and a high voltage VPP is required to drive word lines. This high voltage VPP is generated by a high voltage generation circuit in consideration of a supply voltage VDD and threshold voltage Vth of a MOS transistor constituting the cell transistor. The high voltage generation circuit includes an oscillation circuit, a charge pumping circuit, etc. The oscillation circuit is operated in response to an enable signal to generate an oscillation signal with a certain period, and the charge pumping circuit pumps the supply voltage VDD, which is an external voltage, in response to the oscillation signal to generate the high voltage VPP.
In a memory device having a low supply voltage VDD of about 1.8V, the high voltage VPP is on the order of 3 to 4V. The high voltage VPP is higher than the supply voltage VDD and is generated by pumping the supply voltage VDD using the pumping circuit. In particular, in the case where the supply voltage VDD is low, a high voltage pumping circuit such as a VPP tripler is used because it is difficult to generate the high voltage VPP by pumping the low supply voltage VDD.